Recently, in the manufacturing technology of printing plates for offset printing, a number of CTP systems that directly applies image exposure onto a plate material of a planographic printing plate by a laser light source based on digital data of an image to record the image has been developed and applied to practical application. Particularly, in accordance with the higher output capability in the solid laser and semiconductor laser having emission wavelengths in the near-infrared region, the CTP system for recording by near-infrared light, namely so called thermal CTP system has been widely spread.
As a plate material of a planographic printing plate used for the thermal CTP (the plate material for CTP), U.S. Pat. No. 5,340,699 describes an example of including infrared absorbent, acid forming agent, resole resin, novolac resin as the components of an image recording layer. In this plate material for CTP, acid is generated for an image by the mutual action of the infrared absorbent and the acid forming agent in the laser exposure section, and the acid bridge reaction of the resole resin is developed by heat treatment after exposure. An unexposed portion has high solubility toward the alkali aqueous solution and can be dissolved and removed by alkali processing, while an exposed portion in which the cross linkage has been developed loses solubility toward the alkali aqueous solution and the image recording layer remains imagewisely, thereby an image is formed therein.
Further, in a negative type of image forming material comprising providing the image recording layer including the infrared absorbent, radical generator, radical polysynthetic compound, and binder polymer, there is known an example of carrying out image exposure by infrared laser followed by heating the material at a temperature in the range of 60 through 120° C. for 1 through 20 seconds, namely carrying out the heat treatment after exposure, wherein, for example, triazine compound including trihalomethyl group may be used as the radical generator (see Patent Document 1).
On the contrarily, higher sensitivity is required from the standpoint of effectiveness of plate-making work for manufacturing printing plates, and Publication of a Translation of an International Application No. 2002-537419 discloses a technology in which higher sensitivity is achieved by causing the image recording layer to contain polycarboxylic acid having a specific formation without shortening the material storage time.
Further, a technology is known that raises effectiveness of polymerization and increases the run length capability without carrying out the heart treatment by using a specific cyanine dye as the infrared absorbent (see Patent Documents 2 and 3).
However, when these printing plate materials are manufactured and the plate making work is carried out, the work need to be carried out under a safe light, so that the workability is low, and when this work is carried out under a fluorescent white light (see Patent Document 4), there have been problems in that the dot reproduction is sometimes insufficient, the dot quality of printed matters is sometimes insufficient, and stain recovery capability is sometimes poor.
[Patent Document 1] Japanese Patent Publication Laid-Open No. 2001-175006
[Patent Document 2] Japanese Patent Publication Laid-Open No. 2002-278057
[Patent Document 3] Japanese Patent Publication Laid-Open No. 2003-5363
[Patent Document 4] Japanese Patent Publication Laid-Open No. 2001-194800